When protein is broken down in the body, urea is produced as one of the waste products. Normally, urea is removed from the body by the kidneys and excreted as urine. However, during kidney impairment or failure, urea builds up in the body and particularly in the blood.
When kidneys fail, dialysis is necessary to remove waste products such as urea from the blood. Urea is not very toxic by itself, but its level represents the levels of many other waste products that build up in the blood when the kidneys fail. The need for a patient to undergo dialysis particularly becomes acute in end stage renal disease (ESRD). ERSD causes people to experience a total and irreversible loss of kidney function. ESRD can result from a number of conditions including nephritis, inherited diseases, hypertension and diabetes. In 1997, there were an estimated 230,000 ESRD patients in the United States. That domestic number is growing at approximately 7% annually. Worldwide data indicated that in 1997, there were approximately 157,000 patients in Western Europe, 161,000 in Japan and 230,000 throughout the rest of the world.
Unless a kidney transplant is performed on an ESRD patient, regular blood cleansing is necessary to remove harmful waste products from the blood of the patient to sustain life. Blood dialysis, in particular, hemodialysis is the most common methodology employed to accomplish the removal process. Hemodialysis artificially separates the waste products and excess water from the patient's blood by diffusion and ultra filtration. In hemodialysis, blood is circulated through a machine with a special filter that removes wastes and extra fluids. The clean blood is then returned to the body.
Most ESRD patients go to a clinic—a dialysis center—three times a week for 3 to 5 or more hours each visit. For example, a patient may be on a Monday-Wednesday-Friday schedule or a Tuesday-Thursday-Saturday schedule. The amounts of urea present in the blood, after treatment, can be measured to determine adequacy of the blood filtering procedure. Adequacy is determined by one of two parameters, denoted urea reduction ratio (URR) and dialyzer clearance (Kt/V), respectively.
Although there is no fixed number that universally represents an “adequate” hemodialysis session, it has been shown that patients live longer and have fewer hospitalizations if the URR is at least 60 percent. Considering this variation, some groups that advise on national standards have recommended a minimum URR of 65 percent. The URR is usually measured only once every 12 to 14 treatments or about once a month. The ratio may vary considerably from treatment to treatment.
In the Kt/V measurement, K represents the dialyzer clearance that is expressed in milliliters per minute (mL/min), t is for time, and V is the fluid volume. I is known that Kt/V is mathematically related to URR and is, in fact, derived from it. Kt/V also takes into account two additional factors: (1) urea generated by the body during hemodialysis and (2) the extra urea removed during hemodialysis along with excess fluid. This accounting makes Kt/V a more accurate way to measure how much urea is removed during dialysis than is URR.
However, both the URR and Kt/V techniques currently employed assessments of hemodialysis efficiency of treatment are point measurements, often not even performed at every dialysis treatment, that do not allow continuous evaluation of effectiveness. Both require blood sampling before and after treatment and require time consuming chemical analyses. As a consequence, the measurements cannot be used to determine actual urea clearance efficiency or control the extent and duration of an individual patient's dialysis session.
There is a need for reliable and precise methods for real-time, noninvasive monitoring urea in a patient.